progressive spinal muscular atrophy
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pro·gress·ive spi·nal mus·cu·lar at·ro·phy
one of the subgroups of motor neuron disease; a progressive degenerative disorder of the motor neurons of the spinal cord, manifested as progressive, often symmetric, weakness and wasting, typically beginning in the distal portions of the limbs, particularly in the upper limbs, and spreading proximally; fasciculation potentials are often present, but evidence of corticospinal tract disease (for example, increased deep tendon reflexes, Babinski sign) is not.
spinal muscular atrophy type 1An autosomal recessive condition (OMIM:253300) characterised by degeneration of the anterior horn cells of the spinal cord, leading to symmetrical muscle weakness and atrophy. Disease severity is inversely proportional to the copy number of SMN2, a copy gene which predominantly produces exon 7-skipped transcripts and only low amounts of full-length transcripts that encode for a protein identical to SMN1. Only about 4% of SMA patients bear one SMN1 copy with an intragenic mutation. SMA1 is the severest form of the SMAs, with onset before age 6 months. Infants never achieve the ability to sit.
Defects of SMN1, which encodes a survival motor neuron protein, cause spinal muscular atrophy types 1–4.